1. Field of the Invention
The present invention relates generally to working troughs, and more particularly to a working trough and a method for maintaining a uniform temperature of a working fluid applicable to an electrical discharge machine that performs wire cutting using the working fluid.
2. Description of Related Art
In recent years, technology products with minimized size and high precision have become a trend Minimization and high precision of the products are achieved through a variety of techniques, which are collectively referred to as micro system technologies (MST). The micro system technologies are applied in a variety of fields such as nano-technology, photoelectric communication and biomedical science and technology. To meet the requirements of the micro system technologies and continuously growing applications, micro processing methods of components or dies are strongly needed. When the product size goes into a micro/nano level, product system designs seriously relies on the micro processing capabilities.
Wire-cutting electrical discharge machining (WEDM) is one of a micro processing methods, which melts the work piece and cuts shape by spark discharges occurring when the wire electrode is moved close to the work piece. There are no cutting forces used since there is no direct contact between the wire electrode and the work piece. In addition, no electrode consumption occurs and accordingly there is no processing error caused by electrode consumption. Therefore, the WEDM is an appropriate micro processing method. The WEDM can be divided into an immersion type and a non-immersion type.
For the immersion type, both the work piece and the wire electrode are immersed in deionized water or other appropriate working fluid with high dielectric constant for preventing electrochemical or electrolysis reactions. Further, scraps can be flushed away by the working fluid. Before an immersion type WEDM starts, the working trough is filled with the working fluid. Then, during the machining process, external working fluid in such as a cooling trough is drawn by the upper head of the electrical discharge machine and guided into the working trough along with cycling of the wire electrode, thereby reducing the high temperature generated during the electrical discharge machining and flushing away scraps. Superfluous working fluid flows away from the upper side of the working trough or through overflow holes. FIG. 1 is a diagram showing a wire cutting electrical discharge machine system and FIG. 2 is a simplified view of a wire cutting electrical discharge machine. As shown in the drawings, the system comprises a working trough 1, a cutting wire 101, a lower extending arm 102, an upper head 103, an overflow hole 104, an external fluid storage tank 106, a filter 107, an ion exchanger 108, a cooler 109 and a waste wire box 111. An arrow 105 in the drawing denotes direction of movement of the working fluid. In this technique, the working fluid for machining flows from the upper head 103 into the working trough 1 along with cycling of the cutting wire 101. The superfluous working fluid is discharged through the overflow hole 104, filtered by the filter 107 and cooled by the cooler 109. After the ions of the working fluid are removed by the ion exchanger 108 and possibly additional new fluid (not shown) is added, the fluid flows into the working trough 1 again from the upper head 103 along with the cutting wire 101. The cutting wire 101 extends from the upper head 103, enters into the lower head 110 and passes through the lower extending arm 102 and finally enters into the waste line box 111.
As described above, during operation of the electrical discharge machine, the temperature of the cutting area increases due to electrical discharges, resulting in uneven temperature of the working fluid in the working trough. That is, the temperature of the cutting area varies with time. Therefore, the temperature in this area is in a transient state instead of in a steady state and it reaches a steady state when the temperature of the whole trough reaches a thermal balance with the ambient. In simple words, in the control volume of the whole trough, the temperature of the trough reaches a steady state only when the heat generated by the heat source is equal to the heat dissipated out to the ambient, which however requires a long time. Uneven temperature distribution in the trough and the characteristic of the transient state of the temperature can lead to heat distortion of the lower extending arm, thereby adversely affecting the machining precision. The machining error caused thereby is difficult to be corrected through additional shift or compensation. On the other hand, as the volume of the working fluid guided into the trough during machining is relatively small, the movement in the trough driven by the working fluid guided into the trough during machining is relatively little. As a result, scraps can easily sink to the bottom of the trough instead of being flushed away.
Accordingly, as shown in FIG. 3, Taiwan Patent No. M285409 discloses another wire cutting electrical discharge machine system, which comprises a working trough 2, a cutting wire 21, a lower extending arm 22, a plurality of temperature sensors 23, an overflow hole 24, a plurality of throttles 25, and a cooling control device 26, wherein the temperature sensors 23 senses the temperature of the fluid and transmits temperature signals to the cooling control device 26 such that the cooling control device 26 can control the temperature of the fluid through the throttles 25 or by using frequency conversion technique, thereby maintaining a uniform temperature of the fluid in the working trough. However, the use of the sensors 23, the cooling control device 26 and the throttles 25 increase the cost. In addition, since the positions of the sensors 23 are fixed, phenomenon of uneven temperature can still occur in a short time. Further, the system lacks a mechanism for discharging scraps.
Therefore, how to provide a working trough and a method applicable in a wire cutting electrical discharge machine for maintaining a uniform temperature of the working fluid in the working trough and facilitating the discharge of scraps and having a low cost has become a critical issue for the industry.